In order to make high data rate interactive services such as video conferencing and internet access available to more residential and small business customers, high speed data communication paths are required. Although fiber optic cable is the preferred transmission media for such high data rate services, it is not readily available in existing communications networks, and the expense of installing fiber optic cable is prohibitive. Current telephone wiring connections, which consist of twisted pair media, were not designed to support the high data rates required for interactive services such as video on demand or even high speed interconnects. In response, Asymmetrical Digital Subscriber Line (ADSL) technology has been developed to increase the transmission capabilities within the fixed bandwidth of existing twisted pair connections, allowing interactive services to be provided without requiring the installation of new fiber optic cable.
Discrete Multi-Tone (DMT) is a multi-carrier ADSL technique that divides the available bandwidth of a communications channel such as a twisted pair connection into a number of frequency sub-channels. These sub-channels are also referred to as frequency bins or carriers. The DMT technique has been adopted by the ANSI T1E1.4 (ADSL) committee for use in ADSL systems. In ADSL, DMT is used to generate 250 separate 4.3125 kHz sub-channels from 260 kHz to 1.1 MHz for downstream transmission to the end user, and 26 sub-channels from 26 kHz to 138 kHz for upstream transmission by the end user. Each bin is allocated a number of bits to send with each transmission. The number of bits allocated to an ADSL system are 0, and 2-15 bits.
Prior to transmitting real-time data with an ADSL system, an initialization process occurs. During a first portion of the initialization process, an activation and acknowledgment step occurs. It is during this step that a transmit activation tone is generated following power-up of the ADSL system. Transceiver training is the next step of the initialization process. During transceiver training, the equalization filters of the ADSL system are trained and system synchronization is achieved. Next, channel analysis and exchange are performed as part of the initialization processes. During the channel analysis and exchange, the signal to noise ratio of the channels is determined, and bit loading configurations of the bins and other configuration information are transferred.
Subsequent to the initialization process, real-time data transmission begins. During real-time data transmission, the characteristics associated with the transmission media can change resulting in a varying bit error rate (BER). Two factors that can affect the transmission media characteristics would include temperature changes and a variable noise source. For example, a transmission media can be heated by direct exposure to sunlight, or by mechanical heating generally during start-up when the ADSL transceiver has not reached a steady state operating temperature before the end of the initialization process. An example of a variable noise source would include an adjacent service, such as additional ADSL line cards, which cause interference within the same frequency spectrum.
Changes in transmission media characteristics can affect the overall performance of the ADSL system by affecting the BER of individual carriers. The use of bit swapping has been proposed to maintain system performance when a change in system environment has affected individual carriers. In the prior art, the mean-squared error (MSE) associated with individual bins has been monitored by the receiving portions of the ADSL systems. The MSE represents the error at the decoder of a transmitted data constellation. The MSE, however, is linked only to the constellation for the current number of bits.
The SNR required to successfully transmit data at a specific BER is based on the constellation scheme. Current DMT encoding schemes are not uniform for different size bit allocations, for example, as shown in FIG. 2, i.e. the increase in SNR required to get from a two to three bit transmission is not the same as the SNR increase required to get from a three bit to four bit transmission. With substantially different SNR intervals, judgments based solely on MSE will result in bad swaps in some situations.
Therefore, the bin with the lowest MSE may not be the best candidate to receive a bit. As a result, in the prior art, always swapping from the bin with the highest MSE to the bin with the lowest MSE can result in a lower system performance. In order to address this, the prior art has set a threshold value that must exist between the best and worst case MSE bins that is high enough so that the lowest MSE bin is much better than the highest MSE before a swap occurs. This way, it is guaranteed that the swap will be beneficial. However, a large threshold means that the system performance must be greatly diminished before a beneficial swap can occur. As a result, the prior art system performance is not optimal. In addition, the proposed bit swapping methods of the prior art are not capable of performing bit swapping between bins having zero or two bits associated with them. Therefore, an efficient method of performing bit swapping in a DMT system would be beneficial. Another problem associated with the prior art bit swapping method is that no provisions are provided for bit swapping where Error Correction techniques are used.